Large aircraft are often provided with multi-axle landing gear including a bogie beam carrying a plurality of axles, each axle carrying a plurality of wheels. It is known for one of the axles, generally the rear axle, to be steerable, so as to reduce tyre wear and improve manoeuvrability when the aircraft is taxiing. The steerable axle is generally pivotally connected to the bogie beam. An actuator provided between the bogie beam and the steerable axle can control the orientation of the axle relative to the bogie beam.
As will be appreciated, there are instances where it is desirable for a steerable axle of a landing gear to be locked in a predetermined orientation. Examples are during take-off and landing and during stowage, where it is desirable that the steerable axle is locked in an orientation that is generally orthogonal with respect to the bogie beam and parallel with respect to the other axles on the bogie beam. Such landing gears are therefore provided with a locking device for locking the orientation of the steerable axle relative to the bogie beam.
A known way of providing a locking device is to provide a locking actuator. For example, the steering actuator may be configured to lock when it is at a predefined extension state, corresponding to the extension state required to provide the desired locking orientation of the axle.
A further known locking device is to provide a slot extending into the steerable axle in a manner substantially orthogonal with respect to the longitudinal axis of the axle. A wedge may be provided that is movable between a first position where the wedge is housed within the slot, so as to prevent the steerable axle pivoting about the bogie, and a second position where the wedge is removed from the slot, to allow the axle to pivot.
Both the above-mentioned types of locking device can be noisy during operation because of the fact that unlocking generally occurs under load.
Furthermore, the above-mentioned types of locking device suffer from a disadvantage in that it can be difficult to accurately move the axle to the correct locking orientation before the lock can engage. This can be problematic when the bogie beam bends under static load. As will be appreciated by a person skilled in the art, because the steering actuator is provided between the bogie beam and the steering axle, deflection of the bogie beam can attempt to cause the axle to pivot. This is due to the deflection of the bogie beam causing a change in the straight line distance between the point where the axle connects to the bogie beam and the point where the steering actuator connects to bogie beam. When the lock is released the axle will pivot, and if it moves far enough it may prevent the lock from re-engaging, even though the steering actuator is in its centred position.
Another example of when locking can be problematic occurs if an internally locking actuator is powered to a centred position defined by a control system with a follow-up sensor, or by internal differential area, and the lock position does not exactly coincide with this driven position, due to tolerances, such that the internal lock element does not engage with its mating feature.
In the event of loss of active (electrical) control, an axle will typically move towards its central position due to the pneumatic trail effect or hydraulic means. A locking element of known landing gear may not be able to engage with the axle before the axle passes and overshoots its central position, resulting in the axle oscillating before it is eventually locked.